Generate an older version of dwarf on OSX

[alexcrichton]

OSX apparently doesn't have tooling which understands higher versions of dwarf
by default right now.

Closes #11352

[michaelwoerister]

👏🐤

[comex]

As I described in the issue, clang does this by setting the "Dwarf Version" module flag, and also sets "Debug Info Version", which is required to address a separate issue:

  if (CodeGenOpts.DwarfVersion)
    // We actually want the latest version when there are conflicts.
    // We can change from Warning to Latest if such mode is supported.
    getModule().addModuleFlag(llvm::Module::Warning, "Dwarf Version",
                              CodeGenOpts.DwarfVersion);
  if (DebugInfo)
    // We support a single version in the linked module: error out when
    // modules do not have the same version. We are going to implement dropping
    // debug info when the version number is not up-to-date. Once that is
    // done, the bitcode linker is not going to see modules with different
    // version numbers.
    getModule().addModuleFlag(llvm::Module::Error, "Debug Info Version",
                              llvm::DEBUG_METADATA_VERSION);

Because of the latter, I think it would be better to do the same in Rust, with LLVMAddNamedMetadataOperand, rather than using this patch.

[brson]

Does this fix debugging under lldb?

[alexcrichton]

It did for me on OSX, @comex I'm not sure I quite understand the "correct fix", but I'd be more than willing to review a PR that does to the "correct thing"

[alexcrichton]

Closing in favor of #11864